One embodiment of the present invention relates to semiconductor components and relates to an edge structure for a semiconductor component having two electrodes arranged opposite one another on opposite sides of a semiconductor body, the semiconductor body having at least one pn junction situated in a central region surrounded by an edge region between the two electrodes.
In a conducting semiconductor component of the type mentioned, a drift zone of the p or n conduction type formed in the semiconductor body carries a charge carrier current between the two electrodes. In a blocking semiconductor component, by contrast, the drift zone takes up a depletion zone of the pn junction, which depletion zone forms on account of the high operating voltages compared with the conducting state. In practical application, semiconductor components having a high blocking capability, such as power diodes, power MOSFETs and IGBTs, and in particular those which are used for switching purposes, are intended to have a highest possible breakdown voltage or dielectric strength. In this case, however, the known problem occurs that at high reverse voltages, relatively high electric fields can arise at the surface of a semiconductor component. On account of the field line curvature, this is a problem primarily in the edge region and in particular in curved edge sections (corners) of the semiconductor component, where particularly large excessive field increases may arise on account of the geometrical form. A voltage breakdown therefore takes place principally in the curved corner regions of a semiconductor component.
In order to avoid this problem, a series of measures have been taken into consideration for constructing an edge termination such that the dielectric strength or breakdown voltage thereof is increased.
Thus, the document “Modern Power Devices” by B. J. Baliga, John Wiley and Sons (USA), 1987, pages 92 et seq. and 99 et seq., proposes the use of one or a plurality of floating field rings for this purpose. In this case, in the semiconductor, annular doping zones separated from one another are arranged around the blocking pn junction, and their potential is not defined, but rather is set to a value between the potentials of the two electrodes at which the reverse voltage is present. Said floating field rings are of the same conduction type (charge type) as the diffused zone of the pn junction and of the opposite conduction type to the drift zone of the semiconductor body. The field rings are doped in such a way that they are not depleted in the off-state case and lead to a lateral expansion of the space charge zone and to a reduction of the electric field strength.
Furthermore, the use of field plates is proposed for this purpose on page 116 et seq. of the document cited. This involves layers having a high conductivity, which include metal or doped polysilicon, for example, which are connected to one of the two electrodes and are situated on an insulating layer above the semiconductor surface in an edge region around the pn junction. The field plates carry the potential of the electrode laterally beyond the pn junction and thereby likewise attain a lateral expansion of the space charge zone.
As a further measure for increasing the breakdown voltage or dielectric strength, the use of a “junction termination extension” (JTE) can be gathered from page 113 et seq. of the document cited. The JTE is a lightly doped, lateral continuation of the highly doped part of the pn junction, which is thus doped oppositely to the drift zone. The JTE is substantially or fully depleted in the off-state case and contains approximately the breakdown charge as doping. In the case of the JTE, the doping may also vary laterally, which is then referred to as a so-called “VLD edge” (VLD=variation of the lateral doping). The variation of the lateral doping is described for example in R. Stengl and U. Gösele, IEEE International Electron Devices Meeting Digest, Abstract 6.4, pages 154-157 (1985).
Furthermore, the use of “semi-insulating layers” is known for this purpose from the document “Modern Power Devices” cited above, on page 126 et seq., and also from the document C. Mingues and G. Chariat, IEEE, International Symposium on Power Semiconductor Devices and ICs, Weimar, pages 137-140 (1997). With semi-insulating layers (e.g. SIPOS=Semiisolating polysilicon), the resistance of which is very much higher than that of, for instance, doped polysilicon but much lower than that of silicon oxide, it is possible to realize resistive field plates. These lie on an insulating layer in the region of the semiconductor edge around the pn junction. They are connected to both electrodes and carry a relatively large leakage current according to the reverse voltage present and their resistance. A potential profile determined by the geometrical form and, if appropriate, by an inhomogeneous distribution of the sheet resistance results within the resistive field plate (see EP 0 615 291 A1, Hitachi, “A high breakdown voltage semiconductor having a semi-insulation layer”), which potential profile in turn controls the lateral extent of the space charge zone.
Edge terminations with other resistance layers, for example, made of high-resistance polysilicon, above the edge region around the pn junction function in a similar manner. These layers may be embodied for example in tracks led spirally multiply around the chip in order to obtain a highest possible resistance and thus a low leakage current.
In addition to the abovementioned precautions for increasing the breakdown voltage or dielectric strength, combinations of the abovementioned precautions are also known, such as, for example floating field rings with field plates, field rings which are put at a defined potential by means of a SIPOS layer (see EP 0 615 291 A1), and also the combination of semi-insulating layers with field plates, floating field rings or a JTE (see EP 0 615 291 A1).
In the abovementioned precautions for increasing the breakdown voltage or dielectric strength of semiconductor components, the following problem occurs:
In industrial mass production it is customary to subject components having a high blocking capability to (if possible) nondestructive materials testing prior to delivery to a customer. In this case, the blocking capability of a semiconductor component is normally tested in such a way that a current having a specific magnitude (for example, 1 mA) is imposed on the switched-off semiconductor component (the semiconductor component is then at avalanche breakdown) and the voltage established across the semiconductor component is measured. It has been shown, however, that the semiconductor components can be destroyed very easily in the case of this test method, possibly even at a value of just a few μA, which may be the case particularly for IGBTs with a field stop zone before the rear-side p-type emitter. The destruction location normally lies in the edge region of the semiconductor component, and in particular in one of the corners thereof (“chip corners”), since, on account of the additional curvature of the pn junction that is present in the chip corners, a higher field strength than at the rectilinear (linear) edge sections arises and, consequently, the avalanche breakdown occurs at a lower voltage in the chip corners than in the rectilinear edge sections. In the case of field stop components, a splitting of the current-carrying region takes place at voltage breakdown, moreover, even at a relatively low current density, and this generally leads to the destruction of the semiconductor component.
In practical applications of semiconductor components, often it is not possible to dependably preclude the occurrence of voltage spikes outside a chip's voltage range specified by the manufacturer for the application, as a result of which the semiconductor component can be destroyed principally in its corner regions.